Studies on the pathogenesis of visna of sheep will be continued in this year and aimd at virus interaction with specific cell populations of the reticuloendothelial system and the central nervous system. Immunologic, virologic and pathologic effects of infection will be evaluated. Sera will be examined for the presence of immune complexes. Studies of the neutralizing antibody responses to multiple cross reacting determinants of visna virus will be continued in the coming year. This will be accomplished by immunization of sheep with different strains of visna virus, and absorption of immune sera with similar strains of the agent. By this means we hope to evaluate the role of specific antigenic determinants on the virus in elicitation of anamnestic response to other antigens of the virus.